1. Field of the invention
The present invention relates to a signal generator using a voltage-controlled oscillator (VCO), and more particularly to a signal generator for generating one input signal of a frequency mixing circuit (MIX).
2. Related art
A VCO which is capable of controlling an oscillation frequency by means of voltage is used as a local oscillating source of a receiving system or an intermediate frequency signal source of a transmitting system. The VCO is not used as a single unit but is used by being incorporated as a part of a phase-locked loop (PLL). As for the VCO which is used as the intermediate frequency signal source of the transmitting system, its oscillation frequency is modulated by a modulating signal. Phase-shift keying (FSK) is one desirable system of modulation. In a communication apparatus which combines transmitting and receiving functions, one VCO is used jointly for the transmitting system and the receiving system.
FIG. 4 is a block diagram illustrating a schematic configuration of a frequency conversion portion of a communication apparatus of this type. In this drawing, 1T denotes a transmitting mixer (frequency mixing circuit), and 1R denotes a receiving mixer.
First input signals of these mixers 1T and 1R are both supplied from a VCO 2. A second input signal of the transmitting mixer 1T is an internal local oscillation signal of a fixed frequency. A second input signal of the receiving mixer 1R is a high frequency.
Each of the mixers 1T and 1R outputs a beat signal having a frequency corresponding to the difference or sum of two input signals. An output of the transmitting mixer 1T becomes a transmitting RF signal, while an output of the receiving mixer 1R becomes an intermediate frequency (IF) signal. In a case where the frequency of a received RF signal is high, the output of the receiving mixer 1R becomes a first IF signal.
The VCO 2 is built in a closed loop including a PLL circuit 3 and a loop filter circuit 4, and the oscillation frequency is stabilized. In general, a circuit block including the VCO 2, the PLL circuit 3, and the loop filter 4 is contained in a signal generator called a PLL module 5 in one chip. In the example of FIG. 4, a crystal oscillator which is externally connected to the PLL circuit 3 and an external circuit for inputting a modulating signal for transmission to the VCO 2 are omitted in the drawing.
As shown in FIG. 4, in the configuration of the apparatus in which an output terminal of the conventional PLL module 5 in which the VCO 2 is disposed in an output stage is directly coupled to the input sides of the mixers 1T and 1R, the impedance in a stage following the mixers 1T and 1R is liable to be affected by the output impedance of the VCO 2 and a change in the oscillation frequency.